1. Field of the Invention
The present invention relates to the field of renewable energies and more particularly to the control of wind turbines.
2. Description of the Prior Art
A wind turbine makes possible transformation of the kinetic energy of the wind into electrical or mechanical energy. The wind turbine has the following elements:
A mast is used to place the rotor at a sufficient height to allow for its movement (necessary for the wind turbines with a horizontal axis) or to place the rotor at a height that allows it to be driven by a wind that is stronger and more regular than at ground level. The mast generally houses some of the electrical and electronic components (modulator, control, multiplying year, generator, etc.).
A nacelle mounted at the top of the mast, houses the mechanical and pneumatic components, and some of the electrical and electronic components, necessary for the operation of the machine. The nacelle can rotate to orient the machine in the right direction.
A rotor, which has blades (generally three) and the nose of the wind turbine is fastened to the nacelle. The rotor is driven by the energy of the wind and is linked by a mechanical shaft directly or indirectly (via a mechanical gearbox and shaft system) to the electrical machine (electrical generator, etc) which converts the energy collected into electrical energy.
A transmission, having two axes which are a mechanical shaft of the rotor and a mechanical shaft of the electrical machine is linked by a gearbox.
In the case of offshore wind, a distinction is made between the case where the wind turbine is placed on the seabed (fixed or established wind turbine), and the case where the wind turbine is supported by a platform which floats on the sea and which is anchored to the seabed (floating wind turbine).
Since the beginning of the 1990s, there has been an upsurge of interest in wind energy, in particular in the European Union where the annual growth rate is approximately 20%. This growth is attributed to the production of electricity without carbon emissions. In order to sustain this growth, the efficiency of the wind turbines has to continue to be improved. Wind turbines are designed to produce electricity at a price that is as low as possible. Consequently, the wind turbines are generally constructed to achieve maximum performance at approximately 15 m/s. It is in fact pointless to design wind turbines which maximize their efficiency at even higher wind speeds, since such speeds are infrequent. In the case of wind speeds greater than 15 m/s, it is necessary to lose a portion of the additional energy contained in the wind in order to avoid any damage to the wind turbine. All the wind turbines are therefore designed with a power regulation system.
Increasing wind energy production requires developing effective production tools and sophisticated control tools to enhance the performance levels of the machines. Consequently, the wind turbines are generally constructed to achieve their maximum performance at approximately 15 m/s.
Linear controllers have been widely used for the power regulation by controlling the pitch angle of the blades (orientation of the blades). Techniques that use PI and PID controllers, LQ and LQG control techniques and strategies based on robust linear controls are known.
However, the highly non-linear characteristics of the wind turbine limits the performance levels of these linear controllers. First strategies based on non-linear controls were used. See Boukhezzar B., Lupu L., Siguerdidjane H., Hand M. “Multivariable Control Strategy for Variable Speed, Variable Pitch Wind Turbines” Renewable Energy, 32(2007) 1273-1287.
However, none of these controllers accounts for the mechanical impact (fatigue and extreme moment) on the transmission. Most wind turbine failures are due to breakages or damage affecting the transmission. From data recovered on an offshore application, breakages of the transmission, of the gearbox or of the electrical machine represent nearly 39% of the time when the wind turbine is not producing.